This invention relates to liquid level sensing circuitry and to such circuitry which is particularly suitable for the electronic detection of engine crankcase oil level for determining whether the oil is at an acceptable or unacceptable level.
Monitoring the crankcase oil level for essentially all automobiles and other engine driven equipment presently in use is carried out by the visual inspection of the oil level as indicated on a dip stick that must be manually withdrawn from an engine. Heretofore, gasoline station attendants carried out this procedure for automobiles as a service to customers, but the extent to which that service is available has dwindled due to the establishment of self-service gasoline stations. The burden of carrying out this messy task is left to the automobile operator who is generally reluctant to carry out the task because of its very messy nature. Many times the task is deferred until it can be done without soiling clothing and/or hands or until someone else can do it. This, of course, leads to long periods between oil level checks which in turn results in otherwise avoidable damage to the engine due to the operation of the engine with a low crankcase oil level. While many automobiles have a oil pressure indicator that presents an indication when there is an oil problem, the appearance of such an indication due to a low oil level is usually untimely in that the engine has already been operated at an unacceptable level for a long period of time.
Electronic circuitry is known for the detection of engine crankcase oil level which includes a conductive probe that is in contact with the oil in the crankcase of an engine when the oil is at an acceptable level and is out of contact with the oil when the oil is at an unacceptable level. Such known circuitry recognizes that it is desirable to obtain a signal indicative of the oil level a short time after the engine has been turned off since oil at very low temperatures can give rise to an oil level detection that will produce a low oil level indication when in fact the oil level is acceptable. Such known circuitry uses a timing circuit for producing a clock signal upon completion of a time period after the engine has been turned off to store a signal indicative of a low oil level condition. The stored signal indicative of a low oil level condition is subsequently used when operation of the engine is again initiated to provide a discernible indication of the unacceptable oil level condition. Such known circuitry provides for the immediate entry of a signal indicative of an acceptable oil level which if present at the time an engine is started will provide a perceptible indication indicative of an acceptable oil level. Due to the immediate entry of a signal indicative of an acceptable oil level, it is possible to have an unacceptable oil level indication presented which is changed to an acceptable oil level indication due to oil splashing upon the oil probe upon engine start up causing confusion as to what is to be considered the proper oil level indication. The known circuitry uses logic devices requiring clock pulses for operation which are susceptible to starter motor transient signals, R.F. fields and ignition current spikes giving rise to the entry of erroneous signals. In addition, the known circuitry is complex and does not provide adequate protection for the input device that connects with the conductive probe that is used.